356 EESPIRATION. 



ciently low pressure, or to the action of an insufficient quantity of the 

 reducing action, it gives up a part only of its oxygen ; it is only partly 

 reduced. Such a partly reduced solution still shows the two bands of oxy- 

 haemoglobin. 



290. When the haemoglobin solution (or crystal), which has lost its 

 oxygen by the action either of the air-pump or of a reducing agent or by the 

 passage of an indifferent gas, is exposed to air containing oxygen, an absorp- 

 tion of oxygen at once takes place. If sufficient oxygen be present, the 

 haemoglobin seizes upon sufficient oxygen to obtain its full complement, each 

 gramme taking up in combination 1.59 c.c. of oxygen ; if there be an in- 

 sufficient quantity of oxygen the haemoglobin still remains partly reduced ; 

 or perhaps we may say that a part only of the haemoglobin gets its allowance 

 while the remainder continues reduced. If the amount of oxygen be suffi- 

 cient, the solution (or crystal), as it takes up the oxygen, regains its bright 

 scarlet color and its characteristic absorption spectrum, the single band being 

 replaced by the two. Thus if a solution of oxy-haemoglobin in a test-tube, 

 after being reduced by the action of a drop or two of ammonium sulphide 

 solution and thus showing the purple color and the single band, be shaken 

 up with air, the bright scarlet color at once returns, and when the fluid is 

 placed before the spectroscope, it is seen that the single faint broad band of 

 the reduced haemoglobin has wholly disappeared, and that in its place are 

 the two sharp thinner bands of the oxy-haemoglobin. If left to stand in the 

 test-tube the quantity of reducing agent still present is generally sufficient 

 again to rob the haemoglobin of the oxygen thus newly acquired, and soon 

 the scarlet hue fades back again into the purple, the two bands giving place 

 to the one. Another shake and exposure to air will, however, again bring 

 back the scarlet hue and the two bands ; and once more these may disappear. 

 In fact, a few drops of the reducing fluid will allow this game of haemoglobin 

 taking oxygen from the air and giving it up to the reducer to be played over 

 and over again ; at each turn of the game the color shifts from scarlet to 

 purple and from purple to scarlet, while the two bands exchange for the one 

 and the one for the two. 



291. Color of venous and arterial blood. Evidently we have in these 

 properties of haemoglobin an explanation of at least one-half of the great 

 respiratory process, and they teach us the meaning of the change of 

 color which takes place when venous blood becomes arterial or arterial 

 venous. 



In venous blood, as it issues from the right ventricle, the oxygen present 

 is insufficient to satisfy wholly the haemoglobin of the red corpuscles; the 

 haemoglobin is, to a large extent, reduced, hence the purple color of venous 

 blood. When ordinary venous blood, diluted without access of oxygen, is 

 brought before the spectroscope, the two bands of oxy-haemoglobin are seen. 

 This is explained by the fact that in partly reduced haemoglobin, which we 

 may conveniently regard as a mixture of oxy-haemoglobin and (reduced) 

 haemoglobin, the two sharp bands of the former are always much more 

 readily seen than the much fainter band of the latter. Now in ordinary 

 venous blood there is always some loose oxygen, removable by diminished 

 pressure or otherwise ; the haemoglobin is only partly reduced, there is always 

 some, indeed a considerable quantity, of oxy-haemoglobin as well as (reduced) 

 haemoglobin. It is only under special circumstances, as, for instance, after 

 death by what we shall presently speak of as asphyxia, that all the loose 

 oxygen of the blood disappears ; and then the two bands of oxy-haemoglobin 

 vanish too. If even only a small quantity of oxygen be present, so distinct 

 are the two bands that a solution of completely reduced haemoglobin may be 

 used as a test for the presence of oxygen ; if oxygen be present in any fluid 



